PCF8574

Command Availability:

Available on all microcontrollers, using either hardware or software I2C. I2C2 is not currently supported (as of January 2023).

Explanation:

The PCF8574 library supports the 8-bit quasi-bidirectional port on the PCF8574 via an I2C-bus interface.

The PCF8574 has low current consumption, and includes latched outputs with high current drive capability for directly driving LEDs.

It also has an interrupt line (INT) that can be connected to the microcontroller’s interrupt logic. By signalling on this line, the remote I/O can tell the microcontroller that data is waiting on its ports, without the microcontroller having to poll it over the I2C bus — letting the PCF8574 remain a simple slave device.

The driver supports two commands over I2C:

  • PCF8574_sendbyte( PCF8574_device, out_byte ) — PCF8574_device is the 8-bit I2C address, out_byte is the byte variable to send.
  • PCF8574_readbyte( PCF8574_device, in_byte ) — PCF8574_device is the 8-bit I2C address, in_byte is the byte variable to receive into.

To address a specific slave, define a constant for it, e.g. #DEFINE PCF8574_DEVICE_1 0x4E, where 0x4E is the address identified by I2C discovery.

I2C must be configured (as either software or hardware I2C) before calling either method above.

Example:

    // you must define software I2C or hardware I2C. I2C2 is not currently supported (as of January 2023)
    // Include this specific library. This is mandated
    #INCLUDE <PCF8574.H>

    //! send a variable to the I2C device that sets the device ports
    PCF8574_sendbyte(PCF8574_DEVICE_1, LEDStatus )          ' <<< the PCF8574_sendbyte instruction this page documents

    //! read the status of the port into the variable Portstatus
    PCF8574_readbyte(PCF8574_DEVICE_1, PortStatus )

Key line: PCF8574_sendbyte(PCF8574_DEVICE_1, LEDStatus) — writes LEDStatus to every pin of the PCF8574 addressed as PCF8574_DEVICE_1 in one call, rather than driving each output pin individually.

See Also:

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